In recent years, anaerobic digesters are proliferating in the United States and abroad. The growth of the industry is increasing in popularity as the carbon credits and “manure to power” industry has evolved and grown. The creation of energy (electrical conversion and heat) from methane generated by the anaerobic conversion of organic matter in the wastewater has been documented and is well understood.
There are currently over a hundred operational anaerobic digesters located in the United States and/or a far greater number in Europe, Canada and South America. The industries to which this technology would apply include, but are not limited to, dairies, bovine confinement, porcine confinement and birthing processes, poultry confinement, industrial processes where anaerobic digestion is employed, such as animal processing, food, ethanol and general food processors where a digestible wastewater is generated.
The key process stages of anaerobic digestion include hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The microbial health of an anaerobic digester affects these processes and the overall performance of the digester. Bacterial hydrolysis of insoluble organic material occurs initially, followed by acidogenic bacteria which convert sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria convert organic acids into acetic acid and additional decomposition products ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products into methane and carbon dioxide. Different species of bacteria are able to survive at different temperature ranges. Bacterial that live optimally at temperatures between about 95°-105° F. (35.0° C.-40.6° C.) are called mesophiles or mesophilic bacteria. Some bacteria can survive at hotter temperatures of 125°-135° F. (51.7° C.-57.2° C.) are called thermophiles or thermophilic bacteria.
Anaerobic digestion (AD) may occur in the ambient psychrophilic temperature range routinely observed in the impoundment of treatment lagoons for cows. Conventional anaerobic digesters (ADs) are commonly designed to operate (primarily) in either the mesophilic temperature range or in the thermophilic temperature range noted above. There are usually two reasons why the mesophilic and thermophilic temperatures are preferred. One, a higher loading rate of organic materials can be processed and because a higher retention time (or HRT) is associated with higher temperatures, increased methane outputs for a given digester capacity result. Second, higher temperatures can increase the destruction of pathogens present in the raw manure.
In addition to the temperature, another major consideration is the type of flow through the AD. There are several different types of digesters based on the type of flow, such as fixed film, biphasic orbicular (plug flow and fixed film), upflow anaerobic sludge blanket (UASB), and plug flow to name but a few. One example of a plug flow digester that has met with success is a plug flow digester provided by GHD, Inc. of Chilton, Wis. Details of this digester are disclosed in U.S. Pat. Nos. 6,451,589, 6,613,562, 7,078,229, and 7,179,642. A need, however, exists to improve the digestibility of the solids, to convert more of the organic moieties to methane, and to reduce chemical oxygen demand (COD). Many, most notably, the fixed film digesters, have touted the ability to destruct more solids for the formation of methane, a yield which, theoretically, results in more biogas (methane) per unit of volume and time. However, the theory and the practice do not match and results thus far have been dismal.
Anaerobic digestion can be accomplished with fluid flow of varying types, either direct, as in a conventional plug flow (pushing the solids throughout the length of the digester), an upflow type such as a fixed film (additions form the base flowing up and then down in a directed manner), or sludge blanket types of upflow and directed, only without the use of numerous plates, simply a sludge blanket as the collection area for the bacteria growth and collection. Fixed film and UASB use bacteria growth media and collection to speed up the process. Plug flow uses time as the primary growth medium.
The concentration of solids into a digester is an important factor of the economic decisions on digester type. A high solids, high flow system cannot be used in most digesters, it is simply economically not feasible. For example, a fixed film digester cannot receive more than 7% solids as an influent feed. At 250 gallons per minute assuming a 10% solids feed, and a 7 day retention time, the volume of liquid to be retained is 3,600,000 gallons. A plug flow digester with 21 days retention time requires finding a source of more concentrated waste to bring the solids up to 12%, the recommended concentration for feed to the digester. Under those conditions, the volume to be retained is 7,560,000 gallons. A fixed film digester is constructed of above-ground, glass-lined steel tanks. A plug flow digester is constructed of below ground concrete bunkers. Given the present construction costs for the above ground design for the fixed film and the concrete bunker design for the plug flow, the plug digester may be fabricated at a substantially lower cost per unit of volume compared to the fixed film digester.
Mixing is another of the parameters that affects the performance of the digester. Continuous mixing is important for a variety of reasons, not the least of which is the contact required for the bacteria and the enzymatic reactions to take place. Contact time is a factor, but not nearly as important as the mixing of the solids and the liquid phases. Some anaerobic digester technology employs a gas mixing method, other digester technology use directed flows for mixing and turbulent actions throughout (such as cascading) for the mixing. Additional factors impact the performance of a digester in producing methane, e.g. height of the digester, length, width, gas collection area, etc.
Anaerobic digesters are generally designed and optimized to digest a specific type of source material. Digesters designed for one type of solid waste material usually operate at much lower efficiency when attempting to digest other types of solid waste material. For example, an anaerobic digester optimized to digest one type of waste solid, such as cattle manure, may not be effective to digest another type of waste solid, such as ethanol fermentation waste solids, dairy waste solids, or pharmaceutical biological waste solids. Even subtle changes in the type of source material affect the digester operation and performance. As an example, a digester designed to digest cattle and dairy cow manure will not operate as effectively to digest other types of manure, such as chicken and hog manure.
Healthy and abundant microbes are expected to destruct larger quantities of waste solids and generate larger quantities of methane gas. The microbial health of an anaerobic digester can be affected by variation of source material being digested, microbial nutrition, temperature variations within the digester, pH changes within the digester, and so forth. The effectiveness of an anaerobic digester may be evaluated based upon the amount of methane generated and upon the destruction of waste solids. Digester effluent waste solids are characterized by size, such as suspended solids (<10 μm in size) and the so-called “bedding” solids (>10 μm in size). An effectively operated anaerobic digester produces an effluent with low solids content.
Anaerobic digesters usually operate within a narrow temperature range, which in many geographical locations is hotter than normal ambient temperatures. In cooler seasons, the source material is often at a temperature much lower than the digester operating temperature. As a cool suspension of waste solids (source material) is introduced into the warm solid suspension within the digester, a temperature differential or thermocline may develop. Under such conditions, solids have been known to precipitate or settle within the digester at the beginning of the process. Settling has also been known to occur later on in the digester process by even small temperature differentials within a digester. Such settling can cause short circuiting and channeling of the digester fluids. Short circuiting and channeling substantially reduce and may even prevent effective operation of the anaerobic digester.
In view of the foregoing, it would be an improvement in the art to provide an anaerobic digester that minimizes temperature differentials and associated solid settling, channeling and short circuiting. It would be a further improvement in the field of anaerobic digestion to provide a digester design that improves mixing and temperature control and increases the yield of methane gas generated from the anaerobic digestion of solids.
It will be appreciated that there is also a need in the art for an anaerobic digester that provides effective control and maintenance of microbial health within the digester. It would be another advancement in the art to provide an anaerobic digester that permits effective digestion of a variety of source material waste solids so that the digester may be efficiently used and managed.
A need exists in the field of anaerobic digestion for a digester that has the capacity for increasing the yield of methane gas generated from the anaerobic digestion of solids, either from waste or direct digestion of products and by-products. There is also a need in the art for an anaerobic digester that more effectively destructs waste solids.